The nhc250 is an obsolete engine. It did have a very long service life, so there are lots of parts around still. Some did come with a turbo & some companies do offer a retro fit turbo. It ain't cheap though. The better bet is to hit up a bone yard & pull the parts from a tubo engine. It seems to be a solid motor though. I mainly drove cargo & tractors in the army, like the M931a2 with the 8.3 Cummins turbo diesel. They were real nice at higher elevations where the air is thinner.
So far, this motor seems good. Haven't pulled a load yet though. Hopefully that's gonna change in the next week or two. I will say that the fuel system on these trucks is just flat ass stupid. The 250 is reeeeeeeeally sensitive to air & it pretty much a vacum system. With the factory tank selector & scads of fuel lines there are way to many places for it to suck in air. That was half my problem. The other half is that the truck sat for a long time. The seals in the injection pump had dried out and were sucking air as well. Now I've got the new pump and no tank selector. I'm plumbing in a transfer pump from the passenger tank to the drivers, with a manual switch in the cab. This way there's about twenty fewer spots it can suck air & there's only going to be one rubber line in the feed system. Still a work in progress.